It took be a second to think, but Under Night is BEGGING for an OVA. The characters are all weirdo anime archetypes to begin with, so you'd have Hyde and Linne going on some kinda adventure while dealing with the weirdos of the cast. Some really famous monster animator would be called in to animate Merkava. My boy might not show up, but Akatsuki would definitely be fodder for the third OVA where his time travel jump would be hastily justified for some middle of the episode fightscene where Hyde and Linne have some misunderstanding but Akatsuki's presence is the trigger for a big showdown with that tape's villian.